The present invention relates to vapor generators and more particularly to a support system for bottom supported vapor generating units.
It is customary to support the generating tube banks and the furnace wall tubes of a vapor generator from the drums or headers to which they are connected. Vapor generating units may be top or bottom supported.
A top supported unit is one where the furnace wall tubes and lower headers are supported from the upper headers, and the generating tube banks and lower drum are supported from the upper drum. The upper drum and headers are connected through hangers to a vertical steel structure providing a fixed support. As the top supported unit is brought up to operating temperature, the drums and tube banks and the furnace wall tubes and headers undergo thermal expansion in a downward direction from the point of restraint at the hangers. A top supported unit has the economic disadvantage of larger initial capital investment attributable to the need for higher headroom and the relatively large quantity of steel required by the support structure.
A bottom supported unit is one where the generating tube banks and upper drum are supported from the lower drum, and the furnace wall tubes and upper headers are supported from the lower headers. The lower drum and headers rest on columns positioned therebeneath. As the bottom supported unit is brought up to operating temperature, the drums and generating tube banks and the furnace wall tubes and headers undergo thermal expansion in an upward direction from the point of restraint at the columns. Heretofore, in bottom supported units, it has been necessary to limit the difference in elevation between the lower drum and the lower furnace wall headers so as to avoid excessive stresses on these and associated parts occasioned by relative vertical expansion movements between the furnace wall tubes and headers and the generating tube banks and drums. This limitation imposes a definite restriction on the height of the furnace chamber relative to that of the drums and generating tube banks, and consequently limits the vapor generating capacity of such units for a given width.